The X-ray CT device is configured to calculate an X-ray absorption coefficient at each point in a subject body from measured projection data obtained by multidirectionally taking images of the subject, and acquire the X-ray absorption coefficient distribution image (hereinafter referred to as CT image). Generally, the X-ray absorption coefficient is replaced with a CT value standardized with air and water (air: −1000, water: 0) for diagnostic use.
The CT image is expressed by superposing tomographic planes of the subject along the body axis. The CT image is clinically useful in the medical site because the use of the CT image allows correct and instantaneous diagnosis of the patient condition. Satisfying the condition for high image quality necessary for diagnosis of the physician inevitably involves the subject in radiation exposure to a certain degree. Making efforts to reduce the X-ray dosage for low radiation exposure may increase the ratio between the detected signal and noise. As a result, linear streak artifact and granular noise which may cause diagnostic error may increasingly occur. There has been demanded establishment of both quality diagnosis and low radiation exposure by reducing the streak artifact and noise during image-taking with low radiation dose.
The iterative reconstruction method according to Patent Literature 1 discloses that the calculated projection data or the CT images are iteratively corrected in order to equalize the difference between the calculated projection data and the measured projection data so as to reduce noise. Compared with the generally employed method for analytically calculating the CT value, the iterative reconstruction method is required to cope with more problems of, for example, increase in the calculation amount resulting from repetitive updating, and need for optimization of huge amounts of parameters.
In order to establish noise reducing effect demanded by the smoothing process during updating as one of the problems, many parameters have to be set up. Patent Literature 1 employs the measured value of noise of the CT image output during the iterative correction, in other words, standard deviation (hereinafter referred to as SD) indicating dispersion in the CT values by the iterative reconstruction. The correction is continuously executed, or the parameter for the iterative reconstruction is changed for correction until the measured SD during the iterative correction reaches the required SD.